In the production of steel, as for example, in the operation of a basic open hearth furnace, it is common practice to charge the furnace with a mixture of solid scrap steel and molten pig iron. For example, a mill which rolls its own steel ingots and has its own blast furnaces can effectively use this technique. The molten pig iron or hot metal is transported in and poured from hot metal transfer ladles. Present day transfer ladles generally have a circular or oval horizontal cross-section, and are fabricated from steel plate by riveting or welding and lined with refractory material. The ladle is lifted and tilted by an overhead crane to pour the hot metal, for example, into the mouth of a basic oxygen process steel making furnace. The ladle is lifted by a pair of ladle hooks that are suspended by the overhead crane and are pivotally hooked to a pair of diametrically opposed trunnions that project horizontally from the side of the ladle. The hot metal is poured into the furnace by tilting the ladle with an auxiliary crane hook attached to the rear of and near the bottom of the ladle. The auxiliary crane hook is also suspended by the overhead crane.
The solid scrap metal is transported to the furnace in a horizontally elongated metal scrap box. The scrap is produced from a number of sources. For example, scrap is produced in the pouring of molten metal into ingot molds; it consists of rejected ingots and ingots too short to roll; and scrap is produced when ingots and secondary products are rolled. In general, scrap is dumped into the furnace first, followed by the addition of hot metal. The scrap can be transported to the furnace by an overhead crane which includes an auxiliary crane hook attached to the back end of the scrap box for use in dumping the scrap.
In steel making operations wherein it is desirable to charge the furnace with both hot metal and solid metal scrap, it is preferred to transport both the hot metal ladle and the elongated scrap box with overhead cranes. Present day operations generally employ two different approaches to the design of such cranes. The first of these approaches is to use a four-girder bridge crane employing two trolleys. A larger trolley is mounted for overhead travel on the two outside girders and is adapted for carrying the hot metal ladle or functioning as the main hoist for the scrap box. A smaller trolley which operates underneath the larger trolley is mounted on the two inside girders and is adapted for carrying and tilting the back end of the elongated scrap box or tilting the hot metal ladle. The two trolleys can operate independently of each other with the smaller trolley passing under the larger trolley. An alternative approach to this design is to simply employ two cranes, one being adapted for transporting and pouring the hot metal ladle and the other being adapted for transporting and pouring the scrap box.